Aircraft turbine engines include a turbine module for extracting energy from a fluid stream comprising hot, gaseous products of combustion. The turbine module includes one or more arrays of blades and one or more arrays of vanes. Each blade array comprises multiple blades projecting radially outwardly from a rotatable hub. One array of guide vanes resides aft of the aftmost array of blades. These vanes are referred to as exit guide vanes. During engine operation, the fluid stream flows through the turbine module causing each blade array and its associated hub to rotate about a rotational axis. The rotating blades impart a substantial circumferential velocity component or swirl to the fluid stream, which reduces the thrust output of the engine. The fluid stream discharging from the aftmost array of blades flows through the array of exit guide vanes which deswirls the fluid, causing it to flow in a substantially axial direction thereby restoring thrust output that would otherwise be lost.
Ideally, the exit guide vanes must satisfy several requirements. One requirement, as noted above, is to turn or deswirl the combustion gases coming off the aftmost array of blades so that the gases exit the turbine module in a substantially axial direction. Second, the guide vanes must be able to tolerate changes in the incidence angle of the oncoming gas stream. The incidence angle depends on the circumferential component of velocity imparted to the fluid stream by the blades. This component varies considerably as a function of engine power. In particular, the guide vanes must be able to capture and redirect the gas stream across a wide range of incidence angles without being susceptible to aerodynamic separation and the attendant aerodynamic losses. A third requirement is that the guide vane array must have enough flow capacity to accept the full volume of combustion gases delivered to it. Otherwise the guide vane array would choke the flow through the turbine resulting in a shortfall in thrust. Fourth, in some military applications it is desirable for the guide vanes to block or interrupt an external observer's line of sight to the hot, rotating blades. This helps make the engine and its host aircraft less conspicuous to radar and infrared detection equipment.
It is difficult to concurrently satisfy all these requirements with conventional vanes. A vane having a conventional airfoil cross-section benefits from a large leading edge radius and large leading edge wedge angle which allow the vane to tolerate a wide range of incidence angles without being susceptible to fluid separation. However the large radius and wedge angle constrain the flow capacity of the vane array. Flow capacity can be restored by using a smaller quantity of vanes, however doing so can establish a line of sight to the hot blades, making the engine and its host aircraft vulnerable to detection. The line of sight can be interrupted by using wide chord vanes, but such vanes have the disadvantage of introducing undesirable weight, possibly even more weight than was saved by reducing the quantity of vanes. Alternatively, the line of sight can be interrupted by using highly cambered vanes. However an individual highly cambered vane is susceptible to aerodynamic separation, and an array of such vanes may not have adequate flow capacity.
It may also be possible to satisfy the conflicting requirements by employing variable pitch angle vanes, however this has the considerable disadvantage of introducing additional weight, cost and complexity into the engine.
What is needed is a simple, light weight vane array that exhibits satisfactory aerodynamic performance across a spectrum of operating conditions and that interrupts an observer's line of sight to components residing upstream of the vanes.